This invention relates to an organic light emitting diode (OLED) and, more particularly, to a thin film organic light emitting diode with edge emitter waveguide and an electron injector layer to increase waveguide efficiency.
Thin film organic light emitting diodes typically have an organic electroluminescent medium consisting of two extremely thin layers which are less than 1.0 .mu.m in combined thickness separating the anode electrode and the cathode electrode. One thin film is hole transporting and the other thin film is both electron transporting and also acts as the organic luminescent zone of the OLED device. The extremely thin organic luminescent medium offers reduced resistance, permitting higher current densities for a given level of voltage biasing. The reduced resistance and higher current densities allows applied voltages to be reduced into ranges compatible with integrated circuit drivers, such as field effect transistors. At the same time, light emitted at these low driving voltages has been sufficiently bright to permit visual observation under common ambient lighting conditions.
The OLED contains spaced electrodes separated by an electroluminescent medium that emits electromagnetic radiation, typically light, in response to the application of an electrode potential difference across the electrodes. The electroluminescent medium must not only be capable of luminescing but must also be capable of fabrication in a continuous form (i.e. must be pin hole free) and must be sufficiently stable to facilitate fabrication and to support device operation.
Since the organic luminescent medium is quite thin, light is emitted through one of the two electrodes as a surface emitting OLED. The anode or cathode electrode is a translucent or transparent coating, either on the organic luminescent medium or on a separate translucent or transparent support.
Organic light emitting diodes can be constructed employing thin film deposition techniques. Using an anode as a device support, the organic electroluminescent is deposited as a single film or a combination of films followed by the deposition of a cathode. Thus, starting with the anode structure, it is possible to form the entire active structure of an OLED by thin film deposition techniques. As employed herein the term "thin film" refers to layer thicknesses of less than 5 .mu.m, with layer thicknesses of less than about 2 .mu.m being typical.
Surface emitter OLEDs are not very bright in the surface normal direction. An edge emitter OLED would integrate light from a long emitter area and produce gain in the form of enhanced brightness emitted out of the smaller edge direction. An edge emitter OLED will have an increase in brightness of several times over a surface emitter OLED under similar electroluminescence and voltage conditions.
There are two principal problems with thin film edge emitter OLEDs. First, the thin film electroluminescent layer presents a thin edge or facet to emit the light. Second, the layer surfaces within the OLED structure are lossy, absorbing or blocking the emitted light along the relatively long length of the OLED before the light can be emitted from the edge. Both these problems result in a low intensity emitted light from a thin film edge emitter OLED.
Although surface emitters are conventional in organic light emitting diodes, a thin film electroluminescent line emitter (TFEL) can be an edge emitter, as taught in U.S. Pat. No. 4,535,341 to Kun et al. The thin film edge emitter electroluminescent line emitter has a first dielectric layer, an electroluminescent phosphor layer and a second dielectric layer between a bottom electrode and a top electrode.
The dielectric layers and the phosphor layer have sufficiently different indices of refraction so that light emitted within the phosphor layer will be internally reflected at the first dielectric layer/phosphor layer surface and also internally reflected at the phosphor layer/second dielectric layer surface. The light will eventually be emitted through the edge of the phosphor layer and thus the edge of the OLED.
However, the TFEL has a low propagation efficiency due to grain boundary scattering. The TFEL has waveguiding losses on the order of 400 dB/cm. Furthermore, OLEDs operate at a lower voltage than TFELs and may enable a lower loss light emission device with higher overall optical gain.
It is an object of this invention to provide a thin film OLED with edge emission through a waveguide and an electron injector layer to increase waveguide efficiency.